The present invention relates to a method of heat-treating a steel member.
In exemplary embodiments, steel members such as gears are often subjected to a carburizing and hardening treatment for increasing surface hardness while maintaining toughness. In the carburizing and hardening treatment, carburization is performed whereby the carbon concentration on the surface of a steel member is increased while the steel member is heated at the austenitizing temperature or higher, and hardening is then performed to ensure the toughness of the core part and to increase surface hardness.
In a known carburizing and hardening treatment, a carburizing treatment is performed for an extended period with a large heat-treating furnace including an oil quenching tank disposed at the outlet side. Oil quenching is then performed immediately after the carburizing treatment. Oil is used as a quenching medium in the hardening treatment because oil allows for relatively mild cooling as compared to water quenching, thereby suppressing the generation of distortions.
Since the known carburizing and hardening treatment requires an extended carburizing treatment using a large heat-treating furnace, the processing time is long and a large amount of energy is consumed. Consequently, it is desirable to reduce the total processing time required for the carburizing and hardening treatment, reduce total energy consumption, and reduce the size of the carburizing and hardening apparatus.
Under these circumstances, a method of induction hardening in which hardening is performed for only a part of the member (see Japanese Unexamined Patent Application Pub. No. 11-131133) may be used as a hardening treatment after the carburizing treatment.
According to the descriptions of Japanese Unexamined Patent Application publication Nos. 5-148535 and 8-311607, members are gradually cooled in a predetermined temperature range at a cooling rate of a predetermined value or less after carburization.